Page 258 - Chemical process engineering design and economics

238 Chapter 5

A single-stage turbine is limited to about 2,500 hp (1,490 kW) [31]. The
efficiencies plotted in Figures 5.19 and 5.20 are used for estimating steam flow
rates. Methods for determining more accurate efficiencies and steam flow rates
are given in Reference 5.31.
When higher power than a single-stage turbine can provide is needed, then
use a multistage turbine for greater efficiency and hence steam economy. Turbine
efficiencies for both condensing and noncondensing, multistage steam turbines are
given in Figure 5.21. These efficiencies must be corrected for the effect of using
superheated steam and the discharge pressure, if it is in the vacuum region. Thus,

r|=c s c P r|B (5.45)
where r| is the efficiency of dry saturated steam, obtained from Figure 5.19. The
B
superheat correction factor, GS, and the pressure correction factor, CP, are also ob-
tained from Figure 5.21. In the upper part of Figures 5.19 and 5.21, a half-load,
steam-rate factor is plotted. When the turbine is delivering half its rated power,
the steam flow rate will be equal to this factor times one half the full-steam flow
rate.
The ideal final state, designated with a subscript s, is reached by conducting
an isentropic process from state one to state two. This process is given by Equa-
tionS.11.3 in Table 5.11.
If the steam leaves the turbine part liquid and vapor, the properties of the
exit stream are determined by a mass fraction average of the properties of pure
liquid and vapor as given by Equation 5.11.11 to 5.11.13. According to the phase
rule, these properties are a function of one thermodynamic variable. Because the
inlet steam is superheated, the properties depend on two variables as given by
Equation 5.11.14 and 5.11.17. Problem 5.3 illustrates the calculation procedure
given in Table 5.12.

Example 5.3 Sizing a Steam-Turbine Drive for a Centrifugal Compressor

Superheated steam at 13.0 bar (189 psi) and 260.0 °C (500 °F) is being considered
to drive a compressor. The shaft power required by the compressor, P c, is 100 hp
(74.6kW). If a steam turbine rotates at 3,600 rpm and exhausts at 0.15 bar (2.18
psi), what is the power output, steam rate, and steam condensed.
Follow the calculation procedure outline in Table 5.12. First, obtain the
thermodynamic properties (Equations 5.8.14 to 5.8.21) at the inlet and discharge
of the turbine from the steam tables [44]. These are:
at P] = 1.30 MPa and Tj = 260.0 °C, hi = 2954.0 kJ/kg, s, = 6.8301 kJ/kg-K, and at
saturation T= 191.6 °C

at P 2 = 0.015 MPa, T 2 = 45.81 °C, h 2L = 191.83 kJ/kg, h 2V = 2584.7, s 2L = 0.6493
kJ/kg-K, s 2V = 8.1502 kJ/kg-K

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